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The light-dependent reactions occur in the thylakoid membranes and use light energy to split water (releasing O₂), generate ATP via photophosphorylation, and produce NADPH. Two photosystems are involved: Photosystem II (P680) absorbs light to excite electrons and split water; electrons pass through an electron transport chain to Photosystem I (P700), which uses additional light energy to ultimately reduce NADP⁺ to NADPH. The proton gradient generated by water splitting and electron transport drives ATP synthase in the thylakoid membrane. The ATP and NADPH produced power the Calvin cycle.
Trace electron flow from water through PSII → plastoquinone → cytochrome b6f complex → plastocyanin → PSI → ferredoxin → NADP⁺ reductase → NADPH. Identify where ATP is made and where O₂ is released.
You already know from the photosynthesis overview that plants capture light energy and convert it into chemical energy. The light-dependent reactions are where that conversion actually happens, and they take place in the thylakoid membranes inside chloroplasts. Think of the thylakoid as a solar panel embedded with two specialized light-harvesting machines — Photosystem II (PSII) and Photosystem I (PSI) — connected by an electron transport chain you will recognize from its conceptual similarity to the mitochondrial electron transport chain.
The process begins at PSII, which absorbs light at a wavelength of 680 nm and uses that energy to do something remarkable: split water molecules. This photolysis of water (2H₂O → 4H⁺ + 4e⁻ + O₂) is the source of the oxygen you breathe. The electrons extracted from water are energized by light and passed along a chain of carriers — first to plastoquinone, then through the cytochrome b6f complex, and on to plastocyanin. As electrons move through cytochrome b6f, protons are pumped from the stroma into the thylakoid lumen, building the same kind of electrochemical gradient you studied in oxidation-reduction chemistry. This proton gradient drives ATP synthase embedded in the thylakoid membrane, producing ATP by chemiosmosis — the same principle as in mitochondria, just in a different organelle.
Meanwhile, the electrons arriving at PSI get a second boost of light energy (at 700 nm) and are passed through ferredoxin to the enzyme NADP⁺ reductase, which combines them with a proton to reduce NADP⁺ into NADPH. This is the cell's primary source of reducing power for carbon fixation. The entire electron flow — from water through PSII, along the transport chain, through PSI, and onto NADP⁺ — is called noncyclic electron flow because the electrons travel a one-way path and end up in a new molecule rather than returning to where they started.
The two outputs of the light reactions — ATP and NADPH — are exactly what the Calvin cycle needs to fix CO₂ into sugar. The ratio of ATP to NADPH required by the Calvin cycle is slightly higher than what noncyclic flow produces, which is why cells sometimes run cyclic electron flow around PSI alone, generating extra ATP without making NADPH. The key insight is that the light reactions do not make sugar directly; they convert light energy into the portable chemical currencies (ATP and NADPH) that power carbon fixation in the stroma.